1. Field of the Invention
The present invention relates generally to improvements in optical communication systems. More particularly, the present invention relates to optical communication systems using optical phase conjugation to compensate for fiber dispersion.
2. Description of the Prior Art
Optical communication typically involves transmitting high bit rate digital data over silica glass fiber by modulating a laser or other optical source. Glass fibers have a very broad bandwidth, on the order of 40,000 THz, and can therefore in theory support total data rates on the order of 20,000 Tbits/sec. However, the practical fiber transmission capability is limited by system constraints, among the most important of which are the chromatic dispersion and nonlinearities of the optical fiber itself. Although optical fiber also attenuates the transmitted signal, at a rate of about 0.2 dB per km, the development of erbium-doped fiber amplifiers (EDFAs) has essentially eliminated fiber attenuation as an obstacle to achieving longer transmission distances.
Chromatic dispersion, often simply called dispersion, refers to a phenomenon in which the speed of an optical signal through the fiber varies as a function of the optical signal frequency or wavelength in standard single-mode fibers. For wavelengths below about 1.3 .mu.m, longer wavelengths travel faster than shorter ones, and the resulting dispersion is commonly referred to as normal. Above 1.3 .mu.m, shorter wavelengths travel faster than longer ones, and the dispersion is referred to as anomalous. Dispersion is typically expressed in units of picoseconds per kilometer-nanometer (ps/km-nm), indicating the number of picoseconds a pulse with a bandwidth of 1 nanometer will spread in time by propagating over 1 kilometer of fiber.
One important fiber nonlinearity which can limit transmission capability is the Kerr effect, in which the index of refraction increases with the intensity of the applied optical signal. Changes in the fiber index of refraction modulate the phase of a signal passing through the fiber, and thereby impose a frequency chirp which redistributes the signal frequency spectrum. This phenomenon is known as self-phase modulation in single channel systems in which the optical signal modulates itself. In multi-channel systems, in which one signal causes modulation of other signals, the phenomenon is referred to as either cross-phase modulation or four-photon mixing. Lower frequencies are shifted toward the leading edge of an optical signal pulse and higher frequencies are shifted toward the trailing edge. The resulting changes in frequency distribution are translated to amplitude modulation by the fiber dispersion.
Chromatic dispersion and the Kerr effect therefore both lead to increasing optical signal distortion as a function of transmission distance. For long distance communication over optical fiber, therefore, dispersion and nonlinearities must be controlled, compensated or suppressed. A dispersion and nonlinearity control technique, currently used in terrestrial and transoceanic optical fiber transmission, is electronic regeneration. Repeaters are spaced at appropriate locations along the transmission path to electronically detect, regenerate and retransmit the optical signal before the signal distortion becomes excessive. Electronic regeneration, however, limits the maximum achievable data rate to that of the electronic hardware, rather than that of the wider bandwidth optical fiber. In addition, repeaters are expensive to build and maintain, do not permit flexible system upgradability, and must be spaced at relatively short intervals along the fiber to effectively control optical signal distortion.
Repeaterless compensation techniques have also been developed. One such technique involves solitons, which are optical signal pulses having a well-defined amplitude, pulse width and peak power for a given anomalous dispersion value, such that self-phase modulation due to the Kerr nonlinearity and anomalous chromatic dispersion interact to stabilize the pulse shape. A soliton maintains its shape due to this interplay between dispersion and nonlinearity, and can therefore travel greater distances without regeneration. However, soliton systems also suffer from a number of significant drawbacks, including the need for mode-locked sources and a large number of distributed sliding frequency filters to overcome timing jitter at bit rate distance products on the order of about 100 Tbits/s-km, a need for a large number of distributed amplifiers due to high signal power requirements, a greater sensitivity to amplifier degradation or failure, and difficulty in tracing system failures to a particular portion of the span. These problems generally lead to higher costs for system implementation, maintenance and upgrade. However, soliton transmission provides the highest currently available optical transmission capacity. With sliding frequency filters, a bit rate-distance product of about 200 Tbit/s-km has been demonstrated. This bit rate-distance product will allow, for example, single channel 10,000 km transoceanic transmission at a data rate of 20 Gbit/s. High capacity non-soliton transmission typically requires either non-dispersive fiber or complicated dispersion management, and is limited to a bit rate distance product of about 90 Tbits/s-km.
Several of the problems associated with soliton transmission are alleviated by another known repeaterless dispersion compensation technique, midsystem optical phase conjugation. Because the phase conjugate of an optical pulse is in effect a time reversal of the pulse, an optical phase conjugator placed at the midpoint of a fiber optic transmission path allows the first order chromatic distortion of the first half of the path to be eliminated by the identical first order distortion produced as the conjugated signal propagates along the second half. See A. Yariv, D. Fekete and D. Pepper, "Compensation for channel dispersion by nonlinear optical phase conjugation", optics Letters, vol. 4, pp. 52-54, 1979; K. Kikuchi and C. Lorattanasane, "Compensation for Pulse Waveform Distortion in Ultra-Long Distance Optical Communication Systems by Using Nonlinear Optical Phase Conjugation," 1993 Technical Digest Series Volume 14, Conference Jul. 4-6, 1993, Yokohama, Japan. Midsystem optical phase conjugation has extended the bit rate distance product achievable in the anomalous dispersion region at 1.5 .mu.m wavelength of the conventional single-mode fiber which makes up much of the world's existing fiber communication channels. See A. Gnauck, R. Jopson and R. Derosier, "10 Gb/s 360 km Transmission over Dispersive Fiber Using Midsystem Spectral Inversion", IEEE Photonics Technology Letters, vol. 5, no. 6, June 1993. However, previously demonstrated phase conjugation techniques can achieve a maximum bit rate distance product of only about 3.6 Tbit/s-km, considerably less than that achievable with either soliton systems or non-soliton systems with dispersion management. Thus, the many advantages of optical phase conjugation over soliton or dispersion managed transmission may not presently be obtained for optical communication capacities greater than about 3.6 Tbit/s-km.
Demonstrated optical phase conjugation compensation techniques generally ignore the effects of second order chromatic distortion and nonlinearities in the optical fiber. First order chromatic dispersion is typically approximated as a constant function of wavelength and second order dispersion, its derivative, is therefore taken to be zero. However, in practical systems, second order dispersion is typically on the order of 0.05 to 0.09 ps/km-nm.sup.2. In a linear system, distortions resulting from second order dispersion cannot presently be compensated by optical phase conjugation. The presence of higher order dispersion thus acts as a limit on achievable transmission distance using optical phase conjugation.
As is apparent from the above, a need exists for an improved non-soliton optical communication system which extends the achievable bit rate distance products for non-soliton systems. The improved system should compensate for linear second order chromatic dispersion, for the interplay between first order dispersion and fiber nonlinearities and also for the interplay between second order dispersion and nonlinearities. Furthermore, the improved system should provide bit rate distance products comparable to those presently achievable only with solitons, while avoiding the cost and complexity of soliton transmission.